This invention relates to an ac thin film electroluminescent (AC TFEL) device and more particularly to an AC TFEL device having a white light emitting multilayer phosphor material.
White light emission can be obtained by combining three primary color emissions, for example, by combining red, green and blue light emissions, or by combining complimentary color emissions. Thus, white light emission can be obtained from a single thin film phosphor layer having emissions in the red, green and blue regions of the color spectrum. The emission spectrum of a white light emitting phosphor may consist of either narrow emission bands at appropriate wavelengths for red, green and blue light or a broad emission band extending over the entire visible spectrum. White light emissions can also be obtained by combining phosphor layers, each of which may emit light primarily in a single region of the color spectrum.
Current attempts to produce full color thin film electroluminescent panels typically include fabricating panels having one of two different basic phosphor structures: (a) a patterned structure where stripes of three primary color light-emitting phosphors are deposited side by side on a common electrode/insulator substrate or (b) a layered structure which may include single or multiple phosphor layers emitting either white light, or the three main spectral components of white light, combined with patterned color filters. It is also known to combine the two basic phosphor structures into a hybrid structure having side-by-side patterned red and green light-emitting phosphor stripes on one substrate combined with an unpatterned blue light-emitting layer on a second substrate. In order to fabricate a full color EL device using a broad band white light-emitting phosphor, the broad band emitting phosphor must provide significant emission intensity over a wide wavelength range in order to achieve three sufficiently bright saturated primary colors when combined with suitable color filters.
Tanaka and others, in "Bright White-Light Electroluminescence Based on Nonradiative Energy Transfer in Ce- and Eu-doped SrS Thin Films," 51 Appl. Phys Lett., 1661 (Nov. 1987), report a single layer white light-emitting phosphor, SrS:Ce,Eu,K, which emits electroluminescence over a broad band.
Tanaka and others, in "White Light Emitting Thin-Film Electroluminescent Devices with SrS:Ce,Cl/ZnS:Mn Double Phosphor Layers," 25 Jpn. J. Appl. Phys. L225 (Mar. 1986), disclose a multiple-layered white light emitting phosphor consisting of a greenish-blue light emitting SrS:Ce,Cl and a yellowish-orange light emitting ZnS:Mn.
Ono and others, in "White-Light Emitting Thin Film Electroluminescent Devices with Stacked SrS:Ce/CaS:Eu Active Layers," 66 J. Appl. Phys., 5564 (Dec. 1989), disclose a white light emitting phosphor obtained by stacking layers of blue-green light emitting SrS:Ce and red light emitting CaS:Eu.
Mauch and others, in "ZnS:Mn/SrS:Ce Multilayer Devices for Full-Color EL Applications," SID 93 Digest, 769 (1993), disclose a broad band emitting phosphor consisting of multiple layers of manganese-doped zinc sulfide and cerium-doped strontium sulfide (ZnS:Mn/SrS:Ce), where nine such double layers are employed.
None of these known phosphors exhibits significant emission intensity in the blue region of the spectrum, 450-480 nm. The major peak of cerium emission in strontium sulfide is located at 480 nm. Thus, very little deep blue emission can be obtained through filtering the emission of a cerium-doped strontium sulfide phosphor. Low emission intensity at the 450-480 nm wavelengths will limit the color gamut in the blue region that can be achieved for a color-filtered full color panel and will affect the chromaticity of the combined white color.
Barrow and others, in "A New Class of Blue TFEL Phosphors with Application to a VGA Full-Color Display," SID 93 Digest 761 (1993), disclose a hybrid phosphor structure for a full-color display panel consisting of patterned red light-emitting zinc sulfide doped with manganese (ZnS:Mn) and patterned green light-emitting zinc sulfide doped with terbium (ZnS:Tb) phosphors and a cerium activated calcium thiogallate phosphor layer (CaGa.sub.2 S.sub.4 :Ce) as the unfiltered blue light emitter. However, both the patterned phosphor structure and the hybrid phosphor structure are difficult to manufacture in the very high resolution structures, about 1000 lines per inch, required for head mounted display panels.
What is still needed is a broad band or white light emitting phosphor having improved emission intensity in the blue region and a large color gamut.